Process for preparing tetrafluoro



United States Patent PROCESS FOR PREPARING TETRAFLUORO ETHYLENE BYREACTING CARBON AND A BHIARY HALOGEN FLUORIDE Mark W. Farlow, Holly Oak,and Earl L. Muetterties, Hockessin, Del., assignors to E. I. du Pont deNemours and Company, Wilmington, Del., a corporation of Dela- Ware NoDrawing. Application January 12 1955, Serial No. 481,482

4 Claims. (Cl. 260-653) This invention relates to a new process ofpreparing compounds containing only carbon and fluorine, or carbon,fluorine and another halogen, such compounds being called hereinafterfluorocarbons and halofluorocarbons. More particularly, the inventionrelates to a new process of preparing tetrafluoroethylene.

The fluorocarbons are known to possess considerable usefulness in manyfields of applied chemistry. For example, they have demonstrated utilityas dielectrics, refrigerant liquids and ingredients of insecticidalcompositions (e. g., as propellants). Tetrafluoroethylene has alreadyachieved outstanding commercial success in the form of its polymer. Thehalofluorocarbons are also extremely valuable compounds. For example,the chlorofluorocarbons find extensive use as refrigerant materials andthe bromofluorocarbons as fire-extinguishing liquids.

This invention has as an object a new process for the preparation oftetrafluoroethylene. A further object is a process for the preparationof perhalocarbons containing fluorine. Other objects will appearhereinafter.

These objects are accomplished by the present invention whereintetrafiuoroethylene and other compounds containing only carbon andhalogen are synthesized by bringing a binary halogen fluoride, i. e., abinary compound of fluorine with another halogen, in contact with carbonat a temperature above 1500 C., and removing any unreacted halogenfluoride and any free halogen of atomic number less than 53 from thereaction product as soon as possible after contact with the carbon.

in this reaction, the effluent gas after passage through the reactionzone normally contains, besides the reaction products, unchanged halogenfluoride and the free halogen, other than fluorine, correspondingthereto (fluorine is not present since it reacts instantly with thecarbon). In order to obtain the desired tetrafluoroethylene by thisprocess, it has been found essential to remove these contaminants fromthe reaction product as soon as possible after it leaves the reactionzone. If this is not done, tetrafluoroethylene is not obtained becausethe halogen fluorides and the more reactive halogens (chlorine andbromine) add rapidly to the double bond of tetrafluoroethylene. Thepresence of free iodine in the reaction product can be tolerated sinceit is less reactive with tetrafluoroethylene than the halogens of loweratomic number.

The halogen fluorides suitable for use in the process of this inventioninclude chlorine I fluoride, ClF; chlorine III fluoride, ClF3; bromineI, III and V fluorides, BrF, BrFa, and BrFs, respectively; iodine Vfluoride, IFs, and iodine Vll fluoride, lF-z. These materials, which aregases or low boiling liquids, can be prepared by methods described inthe literature. One of them, iodine V fluoride, TF5, can be made withouthaving resort to elemental fluorine by reacting iodine with silver Ifluoride. For this reason, and also because it leads to good conversionsto halocarbons containing very substantial amounts oftetrafluoroethylene, iodine V fluoride is the preferred startingmaterial for use in this invention.

2,732,410 Patented Jan. 24, 1956 The process can be carried out invarious ways. Thus, the vaporized halogen fluoride can be passed, ifdesired with an inert carrier gas such as nitrogen, argon or helium,through a column of carbon heated at a temperature of at least 1500 C.in a suitable reactor, e. g., a graphite tube placed inside a resistancefurnace or an induction furnace. The gaseous reaction products are thenimmediately treated, as described below, to remove any unreacted halogenfluoride and any free chlorine or bromine, in order to minimize oreliminate the possibility of their reacting with the tetrafluoroethylenepresent in the reaction product. A preferred mode of operation consistsin reacting the halogen fluoride with the carbon electrodes of a carbonarc, where the temperature is estimated to'be in the range of 2500 to3500-4000 C., and again immediately removing from the effluent gas anyhalogen fluoride and free reactive halogen which may be present.'Preferably, the vaporized halogen fluoride is passed through thecarbon" are, for example in an apparatus of the type described below.However, it is also possible to operate with the carbon arc submerged inthe liquid halogen fluoride in a suitably designed apparatus permittingrapid escape of the volatile halocarbons containing thetetrafluoroethylene so that the eflluent gas can be treated at once tofree it from entrained halogen fluoride and halogen.

The removal of the unreacted halogen fluoride and free chlorine orbromine from the reaction product can be accomplished in various ways.The most eflective method consists in treating the efliuent gas,immediately after it leaves the reaction zone, with a chemical agentwhich destroys the halogen fluoride by hydrolysis or metathesis. Suchagents include water, solid alkali metal hydroxides such as potassium orsodium hydroxides, aqueous solutions of the alkali metal hydroxides,carbonates, or sulfltes, and the like. Suitable neutralizing agents forthe unreacted halogen fluoride and corresponding halogen are the metaliodides and particularly the alkali metal iodides, particularlypotassium iodide and sodium iodide, which can be used in the solid stateor in aqueous solutions. These iodides and particularly the iodides ofalkali forming metals, i. e., the alkali metals and alkaline earthmetals are particularly useful when chlorine fluorides or brominefluorides are used. These chemical absorbents are used in amounts atleast stoichiometrically equivalent to the amount of halogen fluorideused for reacting with the carbon. Another method of removing theunchanged halogen fluoride and halogen from the reaction productconsists in cooling it quickly to a temperature low enough to condensethe halogen fluoride and halogen but not low enough to condense thetetrafluoroethylene. This cooling procedure is satisfactory when usingbromine fluoride or iodine fluoride, but with the highly reactive andmore volatile chlorine fluoride it should be replaced or complemented bychemical removal.

Regardless of whether physical or chemical means are adopted to removethe halogen fluoride and free halogen from the reaction product, thisremoval should be done as quickly as possible after eflluent gas leavesthe reaction zone. It is not possible to state, with extreme accuracy,the critical time limit but, for best results as regards the yield oftetrafluoroethylene, it is desirable that the effluent gas be treatedfor halogen fluoride removal within flve seconds after leaving thereaction zone, and preferably within one second. 1

Moreover, conversions to tetrafluoroethylene are improved when theeffluent gas is cooled as quickly as the physical features of theapparatus permit, since rapid quenching minimizes thermal conversion oftetrafluoroethylene to other products. More specifically, it isdesirable to cool the reaction product to a temperature below about 400C. within a very short time after it leaves the reaction zone, 0. g.,within ten seconds and preferably within less than five seconds.

It is desirable to carry out the reaction with an excess of carbonrelative to the halogen fluoride, in order to convert as much aspossible of the latter to fluorocarbons and halofluorocarbons. There issuitably used at least 3, and preferably at least 10, gram atoms ofcarbon per mole of halogen fluoride. A much greater excess of carbon canbe used if desired.

Any form of carbon, whether amorphous or crystalline, is suitable forthe purpose of this invention. Thus, there can be used coal, graphite,charcoal, the various forms of carbon black such as lamp black,acetylene black, bone black, etc. In general, higher conversions areobtained with active carbon, of which many wellknown varieties areavailable commercially. Active car bon is very finely divided, porouscarbon having a total surface area of at least 20 square meters per gram(Hassler, Active Carbon, Chemical Publishing Co. 1951, p. 127). Whenusing the carbon arc, the activity or state of subdivision of the carbonis apparently of no consequence, but the carbon must, of course, possesssufiicient conductivity. The carbon need not be rigorously pure and itmay, for example, contain the normal amount of ash, e. g., from 0.5 to4% by Weight in the case of most active carbons.

The reaction should of course be carried out under anhydrous conditions.It is often desirable to dehydrate the carbon prior to reaction, sincecarbon, especially of the active or absorbent variety, can retainsignificant amounts of water even at high temperatures.

At temperatures below about 1500 C., little or no tetrafluoroethylene isformed, even when the effluent gas is treated immediately for removal ofthe unchanged halogen fluoride. Other halocarbons are formed, and, infact, their production begins at temperatures as low as about 350 C.However, in order to synthesize tetrafluoroethylene in more than traceamounts it is necessary to operate at temperatures above 1500 C.Temperatures in the range of 1500 to about 2300" C. can be obtainedthrough the use of a resistance furnace or of an induction furnace, inwhich is placed a tubular reactor made of corrosion-resistant metal orof graphite. Higher temperatures can be obtained by using the carbonarc, as described in the examples below. The are can be operated at lowor high voltage and with either direct or alternating current. Goodresults are obtained when the reaction is carried out in electric arcsproduced between carbon electrodes with a current of to 50 volts and of10 to 30 amperes, although electric arc operation is by no means limitedto this range of voltage and amperage. Forms of carbon are suitable forthe purpose of this invention are illustrated in detail in applicationSer. No. 409,484, filed by M. Barlow and E. L. Muettertites on February10, 1954, and allowed on October 27, 1954 now U. S. Patent No.2,709,186.

The absolute pressure in the reaction zone is not critical. In general,low pressures of the order of l to 50 mm. of mercury are preferred whenoperating with the carbon arc, since the operation of the arc becomesmore difficult with higher pressures. Atmospheric or lower pressures canbe used when operating with more conventional reactors such as a heatedtube. The rate of passage of the halogen fluoride through the reactionzone should be as rapid as possible consistent with satisfactoryconversions. Times of contact of the halogen fluoride with the carbonwhich are in the range of 0.1 to 30 seconds are suitable.

The reaction gives a mixture of products. Besides tctrafluoroethylene,there is always formed carbon tetrafluoride. In addition, thehalofluoromethanes containing, besides fluorine, the halogencorresponding to the halogen fluoride employed, are present in thereaction product. These include, for example, chlorotrifiuoromethane,

dichlorodifiuoromethane, bromotrifluoromethan'e,dibromodifluorornethane, and iodotrifluoromethane. Higher halocarbons,such as hexafluoropropane and octafluropropane, are formed in smalleramounts. These various products can be separated by fractionation in alow temperature, high pressure still.

The following examples in which parts are by weight are illustrative ofthe invention.

Example I Bromine fluoride, BrFs, was reacted with the carbon electrodesof a carbon arc as follows: The anode was a hollow graphite cylinder, 7inch outside diameter and inch inside diameter, mounted on a coppertube. The cathode was a solid inch graphite cylinder mounted on a coppertube having perforations near the end holding the cathode. The cathodewas positioned with its end nearly flush with the open end of the hollowanode. The electrodes were mounted in a water-cooled, gastight glassjacket which was evacuated to a pressure of a few tenths of a millimeterof mercury. In the operation of this type of arc, the incoming gas flowsout of the perforations in the copper tube around the carbon cathode andenters the hollow anode, passing through the burning arc at this point.The reaction product passes through the anode and out through the coppertube holding the anode.

The arc was operated at a voltage of 20 volts and a direct current of 16amperes. The pressure on the inlet side of the arc was 3.5 mm. ofmercury and that on the outlet side of the arc was 1 mm. of mercury. Atotal of 12 parts of bromine III fluoride was passed through the arc ina period of 10 minutes. The exit gases coming out of the arc were passedimmediately through a trap cooled in a carbon dioxide/ acetone bath,which served the double purpose of cooling the reaction product andremoving from it, by condensation, the bromine and unreacted bromine IIIfluoride present. The residual gas coming out of this cold trap was thencondensed by passage through two traps cooled in liquid nitrogen. Therewas obtained in these last two traps a total of 7 parts of condensedreaction product which was shown by infrared analysis to contain, on amolar basis, 40% bromotrifluoromethane, 45% carbon tetrafiuoride, 1015%tetrafluoroethylene, 1-2% hexafluoroethane, and somedibromodifluoromethane.

Example 11 Iodine V fluoride, 1P5, was reacted with the carbonelectrodes of the carbon are described in Example I. The are wasoperated at a voltage of 16-18 volts and a direct current of 2125amperes, and a total of 3.4 parts of iodine V fluoride was passedthrough it in a period of 5 minutes. The pressure on the inlet side ofthe arc was about 2.5 mm. of mercury, and less than 1 mm. of mercury onthe outlet side. The exit gases were led immediately first through atrap cooled in a carbon dioxide/acetone bath, in which the iodine andunreacted iodine V fluoride condensed and separated from the reactionproduct, then through two traps cooled in liquid nitrogen. The productcondensed in these last two traps (2.5 parts) was shown by infraredanalysis to contain, on a molar basis, 25% carbon tetrafluoride, 20%tetrafluoroethylene, 10% hexafluoroethane, 20% iodotrifiuoromethane,2-5% carbonyl fluoride and traces of silicon tetrafluoride. The carbonylfluoride was presumably formed owing to the presence of residual air inthe reaction zone, and silicon tetra fluoride by the reaction of iodineV fluoride with the glass of the collection system.

Example 111 Chlorine III fluoride, ClFs, was reacted with the carbonelectrodes of the carbon arc described in Example I. In this case, thecopper electrode holders were cooled with water to prevent excessivereaction of the chlorine 1H fluoride with the metal. The are wasoperated at a voltage of 18 volts and a direct current of 25 amperes. Atotal of 2 parts of chlorine III flu :vride was passed through'the arcin a period of 15 minutes. The exit gases from'the reaction was ledimmediately through a tower packed with solid potassium iodide, whichremoved the chlorine and unreacted chlorine III fluoride from thereaction product. The purified gas was passed through a trap cooled incarbon dioxide/ acetone to remove the iodine, then condensed in a trapcooled in liquid nitrogen. The condensate was shown by infrared analysisto contain, on a molar basis, 50% tetrafluoroethylene, 17% carbontetrafluoride, 25% chlorotrifluoromethane, and 8%dichlorodifluoromethane.

In contrast to the above experiment, when chlorine vIII fluoride waspassed through the carbon arc in the same manner but without treatingthe exit gas with an absorbent for chlorine and chlorine III fluoride,no tetrafluoroethylene was found in the reaction product, even when thelatter was quickly quenched by passing it through a trap at 130 C.

The foregoing detailed description has been given for clearness ofunderstanding only and no unnecessary limitations are to be understoodtherefrom. The invention is not limited to the exact details shown anddescribed for obvious modifications will occur to those skilled in theart.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. The process for the preparation of tetratluoroethylene whichcomprises reacting carbon, at a temperature of at least 1500" C. with abinary halogen fluoride and immediately thereafter removing the halogenfluoride and any halogen of atomic number less than 53 from the reactionproduct.

2. The process for the preparation of tetrafluoroethylene whichcomprises reacting carbon, at a temperature of at least 1500 C. with abinary compound of fluorine with another halogen and immediatelythereafter removing the halogen fluoride and any halogen of atomicnumber less than 53 from the reaction product.

3. The process for the preparation of tetrafluoroethylene whichcomprises reacting carbon, at a temperature of at least 1500" C. withiodine V fluoride and immediately thereafter removing the unreactediodine fluoride from the reaction product.

4. In the preparation of tetrafluoroethylene the step which comprisesreacting carbon, at a temperature of at least 1500' C. with a binarycompound of fluorine with another halogen.

References Cited in the file of this patent UNITED STATES PATENTS

1. THE PROCESS FOR THE PREPARATION OF TETRAFLUOROETHYLENE WHICHCOMPRISES REACTING CARBON, AT A TEMPERATURE OF AT LEAST 1500* C. WITH ABINARY HALOGEN FLUORIDE AND IMMEDIATELY THEREAFTER REMOVING THE HALOGENFLUORIDE AND ANY HALOGEN OF ATOMIC NUMBER LESS THAN 53 FROM THE REACTIONPRODUCT.